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(CNN)ISIS forces have retaken the ancient city of Palmyra in Syria, according to Syrian government media, the ISIS media wing and a human rights monitor.

Syrian news agency SANA reported that over 4,000 militants swarmed the city from "various directions," despite having suffered heavy losses from bombardments by the Syrian air force. The Russian Defense Ministry had earlier reported that its aircraft had also taken part in the air campaign.

The UK-based Syrian Observatory for Human Rights (SOHR) confirmed that Palmyra had fallen to ISIS on Sunday after Syrian armed forces pulled out from the desert city, the organization said.

Second occupation

But on Saturday, ISIS had made new inroads in the city, taking neighborhoods and key sites as Syrian troops focused on a ground operation in the city of Aleppo, SOHR said.

Sana added that the jihadist group had transported troops in from Raqqa, its de facto capital in northern Syria, and the city of Deir Ezzor in the country's east.

ISIS fighters had seized almost the entire city Saturday before Russian warplanes began an intense bombardment, forcing the militants to withdraw to orchards and towns on Palmyra's outskirts.

Photos:How ISIS ravaged Palmyra's world treasures

Before: The ruins of the 2,000-year-old Temple of Baalshamin are seen in Palmyra, Syria, in 2007. The ISIS militant group took over the ancient city last year and razed parts of its World Heritage Site. Syrian government forces recaptured the ancient city from the terror group in March 2016. Click through to see the landmarks before and after ISIS' occupation.

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Photos:How ISIS ravaged Palmyra's world treasures

After: A Syrian government soldier walks near what's left of the Temple of Baalshamin on Sunday, March 27. Syrian forces retook the city days before, but damage had already been done by ISIS. UNESCO says it plans to evaluate the extent of Palmyra's damage soon. Many of the structures -- which date from the first and second centuries and marry Greco-Roman techniques with local traditions and Persian influences -- remain in place, bolstering hopes that ISIS didn't completely raze the world heritage site.

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Photos:How ISIS ravaged Palmyra's world treasures

Before: The Arch of Triumph in 2006.

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Photos:How ISIS ravaged Palmyra's world treasures

After: The Arch of Triumph on March 27.

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Photos:How ISIS ravaged Palmyra's world treasures

Before: The Temple of Bel in 2008.

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Photos:How ISIS ravaged Palmyra's world treasures

After: The Temple of Bel on March 27.

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Photos:How ISIS ravaged Palmyra's world treasures

Destroyed statues are seen inside the damaged Palmyra Museum on March 27.

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Photos:How ISIS ravaged Palmyra's world treasures

Many statues in the museum had their faces chipped off -- in keeping with strict Sharia interpretations of the depiction of human forms.

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Photos:How ISIS ravaged Palmyra's world treasures

Before ISIS invaded, authorities took what they could from the museum. But larger items and those fixed to walls had to be left behind.

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Photos:How ISIS ravaged Palmyra's world treasures

Destroyed statues inside the Palmyra Museum.

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Photos:How ISIS ravaged Palmyra's world treasures

The Syrian directorate-general of antiquities and monuments was positive that the condition of artifacts meant that they could be restored and their "historic value" returned, according to a translation of an article on the department's website.

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Photos:How ISIS ravaged Palmyra's world treasures

ISIS took over Palmyra in May.

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Photos:How ISIS ravaged Palmyra's world treasures

By June, ISIS began destroying historical sites.

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Photos:How ISIS ravaged Palmyra's world treasures

Damage inside the Palmyra Museum.

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Photos:How ISIS ravaged Palmyra's world treasures

Palmyra was a caravan oasis when Romans overtook it in the mid-first century. In the centuries that followed, the area "stood at the crossroads of several civilizations" with its art and architecture mixing Greek, Roman and Persian influences, according to UNESCO.

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Russian support

Russia's Defense Ministry said its jets had delivered 64 airstrikes overnight and claimed to have killed 300 militants in the raid.

"During the night, Syrian government forces, actively supported by the Russian Aerospace Forces, repelled all attacks by terrorists on Palmyra. The attacking side actively used car bombs, armor and rocket artillery systems," the ministry told the state-run Sputnik news agency.

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ISIS demolished many of the city's ancient treasures, including the 1,800-year-old Arch of Triumph and the nearly 2,000-year-old Temple of Baalshamin, as well as the Temple of Bel. The group beheaded the antiquities expert who looked after the ruins.

Syria said ISIS also destroyed two Muslim holy sites: a 500-year-old shrine and a tomb where a descendant of the Prophet Mohammed's cousin was reportedly buried.

Palmyra was a caravan oasis when the Romans overtook it in the middle of the first century.

In the centuries that followed, the area "stood at the crossroads of several civilizations," with its art and architecture mixing Greek, Roman and Persian influences, according to UNESCO, the UN agency that documents the world's most important cultural and natural sites.